Method and apparatus for producing fine-grained thermoelectric material

ABSTRACT

System having a carbon ultrasonic transmission line and crucible for obtaining the desired metallurgical effects upon a liquid and a solidifying melt to produce high-purity, high-temperature, fine-grained thermoelectric material wherein abrupt nucleation takes place while the entire melt is constantly being stirred to provide an invariant liquid composition during solidification.

United States Patent Inventors Mirth WM Wayland; Nail: Herman, Lexington, both of, Mass. Appl. No. 715,124 Filed M8. 15, I968 Patented All. 17, 1971 Assignee The United Slates at America a represented by the halted States Atomic Energy Commission mmron AND mm'rus FOR PRODUCING FINE-GRAINED 'nrrm MOELECTRIC MATERIAL 5 Claims, 10 Drawing Ftp U.S.Cl .1 29/5276,

148/129, l64/49, 1 4/11, l64/250, 266/34 BZ3pl7/00 FieldoISe-rdr..... 164/49,

[ 56] References Clted UNI FD STATES PATENTS 2,514,080 7/1950 Mason 340/(8) 2,820,263 1/1958 Fruengel 164/250 2,897,557 8/1959 Omitz H 164/49 3 222,776 12/1965 Kawecki. 164/49 3,363,668 1/1968 Petit et a1 a 1 6 1 4 11 164/49 3,447,587 6/1969 Bodine, Jr. 164/49 Primary Exam1'nerSamuel W. Engle Attorney-Roland Av Anderson ABSTRACT: System having a carbon ultrasonic transmission line and crucible for obtaining the desired metallurgical effects upon a liquid and a solidifying melt to produce high-purity, high-temperature, fine-grained thermoelectric material wherein abrupt nucleation takes place while the entire melt is constantly being stirred to provide an invariant liquid composition during solidification,

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METHOD AND APPARATUS FOR PRODUCING FINE- GRAINED THERMOELECTRIC MATERIAL BACKGROUND OF THE INVENTION In the production of thermoelectric elements, a need exists for a method and apparatus for producing high-purity, hightemperature, doped, thermoelectric elements. Because of its low thermal conductivity, high Seebeck coefficient, and high electrical conductivity, doped PbTe is desirable as the active element but because of the propensity for the doping elements to segregate on normal freezing and also of the inevitable columnar-structure formation on normal freezing, heretofore commercially available PbTe thermoelectric elements have been prepared by cold pressing and sintering and by extrusion of powder charges.

Since these methods have been expensive, burdensome or otherwise subject to difficulties, various other proposals have been made and used to produce such elements, comprising the arrangement shown and described in U.S. Pat. No. 3,l71,009, and while this arrangement is useful, it does require limiting the intended process to the interior of the reaction vessel remote from the walls thereof. It is additionally advantageous to produce homogeneous blocks of machinable fine-grained, stable thermoelectric materials, with grain sizes in the order of magnitude of several microns.

SUMMARY OF THE INVENTION In accordance with this invention, a carbon, low frequency, ultrasonic transmission line and cmcible is used for obtaining the desired high-purity, high-temperature, fine-grained thermoelectric material with high efficiency. More particularly, in one embodiment the carbon member supplies ultrasonic sound energy in the kilocycle/sec. range and concentrates this sound energy in thermoelectric material contained in the carbon member at temperatures up to l,l C. for producing the desired metallurgical results. In another aspect, this invention provides an improved quench-casting process employing a new stationary temperature programming ultrasonic technique that produces suitable grain refinement with a considerable reduction in process time over the systems known heretofore. With the proper selection of elements, frequencies, materials and conditions, as described in more detail hereinafter, the desired fine-grained thermoelectric material and elements made therewith are obtained in the form of a solid block having a suitable shape for further processing or use in the production of thermoelectric power.

The above and further novel features of this invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings, and the novel features will be particularly pointed out in connection with the appended claims.

The invention described herein was made in the course of, or under a contract with the United States Atomic Energy Commission.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings where like elements are referenced alike:

FIG. la is a partial cross section through a first cast thermoelectric structure produced when the surface adjacent to the mold wall is cooled rapidly to the liquidus temperature by heat conduction through the mold wall;

FIG. lb is a partial cross section through a second cast thermoelectric structure;

FIG. is a partial cross section through a third cast thermoelectric structure;

FIG. 2 is a partial cross section ofone embodiment of the carbon concentrator and crucible of this invention;

FIG. 3 is a partial cross section of a practical system for producing thermoelectric materials with the concentrator and crucible of FIG. 2;

FIG. 4 is a partial side view of elements of FIG. 3;

FIG. 5 is a graphic view of the variation in frequency with time in order to maintain the solid-liquid interface at resonance with the apparatus of FIG. 3;

FIG. 6 is another embodiment of the concentrator of FIG. 2;

FIG. 7 is another embodiment of the concentrator of FIG. 2;

FIG. 8 is another embodiment of the concentrator of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is useful in making high-purity, homogeneous, mechanically isotropic, fine-grained, thermoelectric elements displaying chemical, electronic and structural stability at high temperatures and under large temperature gradients, along with relative ductility and machinability. Suitable constituents, comprise suitably doped, P- or N-type isotropic materials, such as N-type Pbl, doped PbTe, or PbTe alloys, such as Pb Sn Te, which because they are isotropic need not employ single-formed crystal material. In fact, the presence of fine grain boundaries in these materials is desirable, since it renders the thermoelement more resistant to creep and low load plastic flow. Brittleness is also an extremely undesirable property for materials subjected to mechanical stress, and in thermoelectric materials in particular, the preferential cleavage so frequently encountered during handling can be catastrophic in operational devices. The ultrasonically produced fine-grained material of this invention, therefore, is particularly resistant to operational mechanical and thermal shock. Additionally, the ultrasonically made finegrained thermoelectric material of this invention can be machined by conventional tooling to high dimensional tolerances.

Ultrasonics involves the production and utilization of sound waves whose frequency range is from the maximum sound frequency audible to the human ear, i.e., from 18,000 to 20,000 cycles/sec. up to a frequency of over 500,000,000 c./sc., as described by B. Carlin in Ultrasonics, McGraw-Hill Book Company, Inc., I949, p. I. To obtain high power outputs, the magnetostriction effect is conventionally employed, as described, for example, in US Pat. No. 2,105,479. Frequencies between 10 and 1,000 kc./s. are convenient, while 15,000 to 30,000 c./s. are common on pg. 220 of the cited Carlin ref.

The use of magnetostriction up to 100,000 c./s., as described on pages 27 and 28 and illustrated in FIGS. 9l and 92 on page 228 of the above-cited Carlin reference, utilizes a magnetized rod or bar of suitable magnetostrictive material. A coil around the rod is energized by passing a current therethrough and the rod will vibrate at the frequency of the applied voltage and with an amplitude roughly proportional to the voltage magnitude, whereby ultrasonic wave energy is produced in the rod by the magnetostriction effect and passes into the material attached to the rod.

As described in the above-cited Carlin reference, ultrasonic power outputs are high from magnetostriction devices, and a 2,000 watt ultrasonic magnetostriction generator was designed and is used in the preferred embodiment of this invention. Other conventional, commercially available, ultrasonic magnetostriction generators that can be used, com prise the Raytheon ultrasonic power generator Model 2-332- 1 which supplies 700 watts of ultrasonic power at a frequency of 25 kc.p.s. This generator has a frequency range between I? and 27 kc.p.s., although efficient operation occurs between :2 kc.p.s. The frequency of the tuning circuit can be manually adjusted to the load frequency. The unit is also equipped with a so-called "resonance frequency meter" and a manual power level adjusting knob. The frequency is also continuously monitored with a digital frequency readout, such as the Berkeley Scientific Corp. Model 554.

The invention hereinafter described utilizes a conventional magnetostriction system of the type described. The ultrasonic energy produced thereby is transmitted through a carbon transmission line forming a crucible for solidifying a melt contained therein, such as the PbTe described above, to produce a chemically and mechanically stable, homogeneous. high-purity, uniformly fine-grained, machinable, cylindrical block of thermoelectric material having optimum electric properties.

In order to explain how the method and apparatus of this invention accomplish the function of producing fine-grained thermoelectric material, reference is made to FIGS. la, 1b and 10, which illustrate the three cast structures, depending on heat transfer conditions, that can occur when a liquid composition of the described materials solidifies in a mold. FIG. la shows the cast structure produced when the surface of the liquid adjacent to the mold wall cools rapidly to the liquidus temperature by heat conduction through the mold wall and begins to supercool. At some degree of supercooling, nuclei of solids form with random orientation. Because of the anisotropy of growth rate, crystals whose direction of maximum linear growth is parallel to that of heat flow grow faster and crowd out crystals of less favorable orientation. Thus, a second zone-called the columnar zone-with a preferred orientation forms adjacent to the chill layer. Depending on the heat transfer conditions, the columnar zone may extend to the center of the casting or, more frequently, be followed by another zone of equiaxed, randomly orientated grains in the center position of the casting, as shown in FIG. lb. These ascast structures described are undesirable since there will be a tendency for planes of weakness to develop where columnar grains growing from the different mold faces meet; of the columnar grains; and there will exist a bulk inhomogeneity due to segregation at dendritic boundaries within the columnar grains and at the columnar grains boundaries.

The type of ingot structure in which the mentioned effects are minimized is a fine-grained, equiaxed structure, shown in FIG. 1:. This fine-grained equiaxed structure is most desirable since it reduces a segregation of alloy constituents; provides better dispersion of any nonmetallic that might be present; reduces the possibility of hot tearing, i.e., separations of grains due to the melting of a low melting point constituent in the grain boundaries; and it imparts a somewhat higher strength.

Referring to FIG. 2, a preferred embodiment of this invention, comprises a high-purity, high-density, graphite concentrator 11 having a sleeve-shaped carbon crucible 12 and a removable carbon top 13 forming a vent 14. The concentrator, which has a tapered portion 15 connected to receive ultrasonic energy from the described transducer and a round portion 16 forming an ultrasonic amplifier horn with the tapered portion 15, transmits the ultrasonic energy from the described generator to a suitable melt 18 of the described thermoelectric constituents to produce the desired fine grained thermoelectric material.

The metallurgical effects of the apparatus and method of this invention, comprise degassing, i.e., elimination of gas from liquids and casting grain refinement, i.e., reducing grain size; homogenization, i.e., uniform mixing; and fine disper' sions of one metal into another or dispersion of bubbles and impurity and oxide inclusions. Also, the carbon member of this invention is considerably more economical from the standpoint of material and machining costs than other materials, such as quartz, particularly in scaled-up versions thereof; the individual carbon crucibles can be machined to conform to the end of the carbon concentrator, in which case the crucibles are reusable; specialized or complicated materials preparation and long processing time is eliminated; and the ultrasonically cast ingots can be easily removed from the carbon crucible, thus eliminating cutting away of the crucible and ingot from the concentrator and avoiding possible mechanical damage to the ingots.

Additionally, the graphite concentrator and crucible of this invention are inert and commercially obtainable in high purity, withstand high temperature, do not react with liquid or melt being processed at high temperatures, are not severely wet by the molten material being processed at high temperatures, possess sufficiently high fracture or cleavage strength at high temperatures to withstand mechanical stress produced by ultrasonic vibration, and possess low thermal expansion relative to the melt.

Commercially available, high-density, high-purity carbon was found to provide these features and advantages, as well as ease of machinability and ability to form a strong epoxied joint with other common materials. It has also been found to be an effective conductor of ultrasonic energy in the 20 kc. 15 kc. frequency range at temperatures of up to l,l00 C. In order to maximize the amount of ultrasonic energy transferred to the melt interface a combination straight and tapered concentrator is used to achieve minimum mass and maximum amplification. This small mass is desirable since it minimizes energy losses due to attenuation in the concentrator material and reduces heat transfer through the concentrator by conduction.

Advantageously, the resonant frequency of the concentrator 11 is matched to the transducer resonance. To this end, with a concentrator 11 that is l inch long, for example, this frequency is determined by the fundamental relation l=n (V/2j), where f= resonance frequency, and v velocity of propagation of ultrasonic waves in the material.

As shown in the practical embodiment of FIG. 3, a metal extension 19 is attached to the concentrator at joint 20 and by stud 21 to the transducer 22, which is suitably connected to generator 23. The extension 19 consists of a half wavelength of straight R-Monel, 2-in. diameter, 3.61 in. long (V,,,,,,,, L804 l0= inch/sec.,F25 kc.p.s.). Since the carbon concentrator 11, which forms an ultrasonic amplifier horn, cannot be attached directly to the flat top %-in. diameter of the transducer 12 by a mechanical coupling, the monel section 19 provides a rigid coupling between the transducer 22 and the car bon concentrator II by means of an epoxy-bonded joint 20. Suitable epoxies, comprise polyglycidyl novolak resins or diapoxides, such as resorcinol diglycidyl ether having suitable curing agents, such as boron trifluoride-amine, monoethylamine or triethanolamine complexes, and suitable catalysts, such as dicyandiamide, cyanamid or melamine.

In the practical system of this invention, which is shown in FIGv 3, the concentrator ll tapers from a ifi-in-diameter cross section to a Lin-diameter cross section. The propagation velocity (with the grain) of the ultrasonic waves at 25 kc.p.s. for the carbon used was l2S l0 inch/sec. The tapered portion 15 may consist of two half-wave sections of carbon (about 5 in. long) and the transition from the tapered to straight section is at antinode of the standing wave in the graphite to minimize stress concentration in the graphite. The section 16 of the concentrator is a straight rod 1 in. in diameter, 7.5 in. long (equivalent to 3 half-wave sections). The total of the 5 half-wave sections of carbon is the allowable minimum length for preventing overheating of the epoxy joint (e.g. C.). However, a water-cooling system 33, as shown in FIG. 4, can be used to cool the epoxy sufiiciently to prevent structural failure thereof.

This cooling system 33 also provides a low load on the vibrating parts of the ultrasonic assembly and acts as the seal base for the vacuum enclosure 35. To this end silicon O-rings, such as rings 37 and 39, provide the only physical contact between the vibrating concentrator 11 and the stationary parts. Prior to each ultrasonic casting run, the quartz bell jar 35 is removed, An electric waterflow switch (not shown) automatically assures circulation of water through pipes 40 and water jacket 41 around the transducer before the ultrasonic power can be turned on.

As will be understood in more detail hereinafter, the charge 18 is directionally solidified and the rate of advance of the solid-liquid interface is dependent on a preferred rate of applied temperature programming. In this regard, the resonant frequency is directly affected by the position of the interface, and for best results, it is necessary to maintain the resonant frequency (manually or automatically) during the solidifica tion process. Thus the rate of interface movement can be accurately determined from the rate of change in resonant frequency during the process. Also, the start and termination of solidification can be detected by the frequency response. This temperature programming can be obtained automatically by utilizing suitable circuits designed for this purpose, in

which case the temperature programmer is normally placed in the temperature control circuit of the heat source 53.

In operation, the ultrasonic energy introduced into the melt 18 in carbon crucible 12 of concentrator ll, is introduced into the melt by concentrator 11 at a resonant condition. This frequency depends on temperature changes in the concentrator assembly and the length of the solidified ingot, which as will be understood from the above, is directionally solidified. To this end an amplitude monitor 43, comprising a magnetized pin 45 attached to the base of extension 19 for the tapered portion of the concentrator l l, is positioned freely inside an accelerometer coil 47, and the heater input is decreased manually or automatically in correspondence with the advancing solid-liquid as indicated by the monitor 43. The mV output of the coil 47 is fed into a sensitive AC electronic voltmeter 49. This provides an excellent measure of relative amplitude of vibration of the concentrator ll and the resonant frequency can thereby be maintained during the entire casting run by automatic or manual adjustment of the frequency of the magnetostriction generator 23 to obtain the maximum deflection of the mV indicator 49.

FIG. 5 shows the variation of frequency required to maintain the solid-liquid interface at resonance during an ultrasonic casting run. As shown in this figure, the generator frequency remains constant before the start of solidification and after the entire melt 18 is solidified, but this frequency decreases steadily during the solidification process. Consequently, this frequency is decreased gradually during solidification to maintain resonance.

In one example of the operation of this invention, a carbon crucible 12 having a Hi inch 0D, 1 inch LD. and 3 inch length is provided at the top of the carbon concentrator ll and is charged with approximately 100 grams of precast thermoelectric material 18 prepared directly from high purity elements. In practice, the appropriate quantities of high purity (99.999 percent) elemental constituents and dopant e.g., N- type PbTe with about 0.4 percent excess Pb-l-0.03 mole percent Pbl,, or P-type material (PbTe) (SnTet are weighed with an accuracy of 0.l mg., placed in the crucible, evacuated to mm.-Hg., sealed and placed in a l3-kw. resistance furnace 53 with an 8 inch long flat temperature zone at l,000 C. To this end the crucible is formed by the top of concentrator II, the carbon cap 13 is fitted on top of the crucible l2, quartz bell jar 35 is placed on a flat rubber gasket 61 as shown in FIG. 4, and is tightened down to the water jacket 41 by fastening screws 62 through a metal collar 63 into the transducer housing 64. The closed chamber 65 in bell jar 35 evacuates and flushes a number of times prior to establishing in the chamber 65 a slight positive pressure of purified Argon through pipes 66 from a suitable pump and gas pressure source (not shown).

A conventional raising and lowering means 67 for heater 53, raises and lowers the heater for melting the charge 18 by lowering the heater 53 to a predetermined position to enclose the charge 18 and a portion of the concentrator 11. For example, a suitable hoist may be used. This eliminates the need for complicated drives having precision slow-speed worm gears, variable speed motors, controllers and gear reducers. Also, the conventional heaters may be used.

A standard platinum platinum 13 percent Rh thermocouple inside the furnace, connects with a standard precision point temperature controller, such as a Honeywell Model 0687078l, for controlling the temperature produced in chamber 65 by the heater. This system is capable of maintaining the furnace temperature within $0.5 C.

The furnace 53 produces a predetermined temperature at the bottom interface of the melt l8 and the heater monitor 43 indicates the interface temperature during the entire ultrasonic run. The charge 18 normally has a fixed temperature (50 C. superheat) for 20 minutes before ultrasonic agitation.

The generator 23 tuned to the resonant frequency, agitates the melt 18 in order to mix and homogenize the charge prior to solidification. In general, the same amount of charge of N- type material requires 25 percent higher energy density to produce grain refinement compared to P-type material. This effect is thought to be related to the difference in sound velocity in the two types of material. However, in both cases the system of this invention produces abrupt, fine-grainproducing nucleation while the entire liquid melt portion is constantly being stirred to provide an invariant liquid composition during solidification.

It was found that by gradually changing the stationary furnace temperature profile and by changing the resonant frequency in the range from 17 to 24 kc./s. to provide directional solidification, that more uniform and finer grain structure can be obtained than by relative withdrawal of the charge from the constant temperature zone in the furnace 53, e.g., at rates between fa-l inch/hr. while resonance is maintained. Since the carbon concentrator l1 basically constitutes a heat sink for the molten charge 18 during solidification, the desired directional solidification initially takes place from the bottom of the crucible 12. Best results have been obtained by utilizing a relatively fast rate of cooling between l0l5 C ./minute under ultrasonic agitation. This technique represents an important step in reducing the complexity of the required equipment 67 and process operation and the time necessary for solidification.

This technique of ultrasonic casting requires the charge to be positioned inside the furnace and permits manual or automatic programming of the furnace temperature during the agitation process. To this end, the start and completion of the charge solidification are detected by sudden changes in amplitude by a suitable controller 69, or controlled by hand, to accomplish the gradual change in the temperature profile imposed on the entire charge [8.

In six ultrasonically cast ingots prepared by this new casting technique, the crucible 12 was held stationary relative to the furnace 53 during the entire run and the furnace temperature was automatically programmed down by controller 69 while agitation progressed. The start and completion of the charge solidification, as detected by a sudden change in frequency and amplitude by monitor 43, was provided by a controlled cooling rate within the range of 5 to l0 C. per minute. The optimum melt temperature corresponds to a superheat of between 50"80 C., since a superheat of less than 50 C. produced excessive cracking and a melt superheat of over C. results in extensive vaporization losses.

The satisfactory use of this new technique has been demonstrated in actual tests, specifically in the case of PbTe and PbTe base alloy thermoelectric P-and N-type material. Equiaxed grain refined ingots of thermoelectric materials weighing up to 500 gm. and 2 inches in diameter have been produced by the application of this technique. A uniform grain size of 70p, in diameter was produced in these ultrasonically refined ingots.

Advantages of this technique are that the equipment required is economical and compact; complicated mechanical parts for transverse movement of the furnace and/or melt are eliminated; the required temperature gradient can be easily maintained throughout the time the temperature is being programmed; the freezing velocity can be controlled over a wider range of temperatures (especially in the low temperature range) much more accurately than in a mechanically movable assembly; and the heat transfer change in the concentrator is minimized since the temperature profile remains stationary throughout the process.

In another embodiment shown in FIG. 6 the crucible 12 contains a suitable liquid material 70 for transferring ultrasonic energy from the carbon concentrator ll to the melt 18 in the crucible 12.

in another embodiment shown in FIG. 7 the melt I8 is contained in a sealed removable crucible 73.

in still another embodiment shown in FIG. 8 the round section 16 of the carbon concentrator 11 forms the crucible 12 in one piece. Advantageously, the walls 71 of the crucible are inclined to facilitate withdrawal of the solidified ingot.

The ultrasonic system of this invention produces homogeneous, machinable, high-temperature, high-purity, fine-grained ingots of thermoelectric material that shows no signs of deterioration after extreme temperature cycling. Moreover, the carbon transmission line and crucible of this invention are simple, compact, practical and efi'lcient and can be used with conventional, commercially available materials and equipment.

What we claim is:

1 Apparatus for producing strong, fine-grained thermoelectric material, comprising an ultrasonic generator having a transducer head, a metallic extension, and means, consisting of a carbon ultrasonic concentrator secured to said metallic extension, a carbon ultrasonic transmission line, and a carbon crucible forming a mold at one end of said transmission line for containing a liquid thermoelectric material melt for transmitting ultrasonic energy seriatim from said generator, through said transducer head, metallic extension, carbon concentrator, carbon transmission line, and carbon mold for constantly stirring said melt for providing an invariant liquid composition therein for solidification thereof in said mold to produce said fine-grained thermoelectric material by cooling the same in said mold while said ultrasonic energy is conducted thereto through said mold, said mold remaining unreactive with said material and unwet thereby at high temperature, and said mold also possessing low thermal expansion relative to said material for separation from said material, and high fracture strength for reuse at the high mechanical stresses produced by said ultrasonic energy therein.

2, The invention of claim 1 in which said transmission line comprises a tapered portion (15) connected to said metallic extension and a round portion (16) exposed to said melt at the bottom of said crucible for forming an ultrasonic amplifier horn that minimizes the mass of said concentrator for maximizing amplification of said ultrasonic energy transmission from said generator to said melt and reducing heat transfer through the concentrator by conduction.

3. The invention of claim 1 in which said crucible comprises a hollow, removable, cylindrical, carbon sleeve that fits around the outside of said carbon ultrasonic transmission line.

4. The invention of claim 1 in which said crucible is formed as an integral part of said concentrator.

5. The method of forming fine-grained thermoelectric elements, comprising the steps of:

a. heating PbTe thermoelectric material to l,l00 C. in an evacuated, stationary, carbon mold forming at one end a heat sink for said material and an ultrasonic concentrator at the other end for effectively conducting ultrasonic energy in the 20 kc./s. fi kc./s. range to said material at l l 00 C.;

b. transmitting seriatim said ultrasonic energy to said material at said temperature through said concentrator and said carbon mold while simultaneously cooling a portion of said concentrator in order to minimize heat transfer from said material to said concentrator;

c. lowering the temperature of said material at a rate of 10-l5 C./minute while continuously changing the frequency of said ultrasonic energy transmitting to said material by said mold for the fine grain solidification of said material in said mold;

d. detecting the start and completion of said solidification from the sudden change in frequency and amplitude of said ultrasonic energy conducted to said material;

e. removing said solidified material from said mold after the completion of said solidification in said mold for the reuse of the same, and;

f. finishing said solidified material for use as a thermoelectric element by machiningv UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3 599,319 Dated August 17 1971 Patent No.

Martin Weinstein et a1. Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Sheets 1 to 5 of the drawings please remove security markings on each sheet of drawings.

Signed and sealed this 10th day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents USCOMM-DC scan-Poe ",5. GOVERNMENT PRINTING OFFICE: I!" S5334 

2. The invention of claim 1 in which said transmission line comprises a tapered portion (15) connected to said metallic extension and a round portion (16) exposed to said melt at the bottom of said crucible for forming an ultrasonic amplifier horn that minimizes the mass of said concentrator for maximizing amplification of said ultrasonic energy transmission from said generator to said melt and reducing heat transfer through the concentrator by conduction.
 3. The invention of claim 1 in which said crucible comprises a hollow, removable, cylindrical, carbon sleeve that fits around the outside of said carbon ultrasonic transmission line.
 4. The invention of claim 1 in which said crucible is formed as an integral part of said concentrator.
 5. The method of forming fine-grained thermoelectric elements, comprising the steps of: a. heating PbTe thermoelectric material to 1,100* C. in an evacuated, stationary, carbon mold forming at one end a heat sink for said material and an ultrasonic concentrator at the other end for effectively conducting ultrasonic energy in the 20 kc./s. + or - 5 kc./s. range to said material at 1,100* C.; b. transmitting seriatim said ultrasonic energy to said material at said temperature through said concentrator and said carbon mold while simultaneously cooling a portion of said concentrator in order to minimize heat transfer from said material to said concentrator; c. lowering the temperature of said material at a rate of 10* -15* C./minute while continuously changing the frequency of said ultrasonic energy transmitting to said material by said mold for the fine grain solidification of said material in said mold; d. detecting the start and completion of said solidification from the sudden change in frequency and amplitude of said ultrasonic energy conducted to said material; e. removing said solidified material from said mold after the completion of said solidification in said mold for the reuse of the same, and; f. finishing said solidified material for use as a thermoelectric element by machining. 